How do I compare a character to check if it is null?

Default value to char primitives is 0 , as its ascii value. you can check char if it is null. for eg:

char ch[] = new char[20]; //here the whole array will be initialized with '\u0000' i.e. 0
    if((int)ch[0]==0){
        System.out.println("char is null");
    }

You can use the null character ('\0'):

while((s.charAt(j)=='\0')

Correct way of checking char is actually described here.

It states:

Change it to: if(position[i][j] == 0) Each char can be compared with an int. The default value is '\u0000' i.e. 0 for a char array element. And that's exactly what you meant by empty cell, I assume.

Check that the String s is not null before doing any character checks. The characters returned by String#charAt are primitive char types and will never be null:

if (s != null) {
  ...

If you're trying to process characters from String one at a time, you can use:

for (char c: s.toCharArray()) {
   // do stuff with char c  
}

(Unlike C, NULL terminator checking is not done in Java.)